FIG. 15 shows a known example of such power supply system (refer for example to Literatures 1 and 2). As shown in FIG. 15, a power supply system 1 includes a power supply part 3 as a power supply unit and a power reception part 5 as a power reception unit. The power supply part 3 includes a power supply side loop antenna 32 to which power is supplied and a power supply side helical coil 33 as a power supply side coil electromagnetically-coupled to the power supply side loop antenna 32 and arranged apart from and opposed to the power supply side loop antenna 32 along a center axis direction of the power supply side loop antenna 32.
The power reception part 5 includes a power reception side helical coil 51 as a power reception side coil arranged apart from and opposed to the power supply side helical coil 33 along a center axis direction of the power supply side helical coil 33 for electromagnetic resonance, and a power reception side loop antenna 52 arranged apart from and opposed to the power reception side helical coil 51 along a center axis direction of the power reception side helical coil 51 and electromagnetically-coupled to the power reception side helical coil 51. Once power is transmitted to the power supply side helical coil 33, then this power is transmitted wirelessly to the power reception side helical coil 51 through electromagnetic resonance.
Once the power is transmitted to e power reception side helical coil 51, this power is transmitted to the power reception side loop antenna 52 through electromagnetic induction and supplied to a load such as a battery connected to the power reception side loop antenna 52. According to the above-mentioned power supply system 1, the power can be supplied in a non-contact faction from the power supply side to the power reception side through electromagnetic resonance between the power supply side helical coil 33 and the power reception side helical coil 51.
Furthermore, by providing the power reception part 5 to a motor vehicle 4 and providing the power supply part 3 to a road 2 and such, the power can be supplied to a battery mounted to the motor vehicle 4 in a wireless fashion by using the power supply system 1 described above.
In the power supply system 1, the impedance of the power supply part 3 and the power reception part 5 is adjusted (i.e. the impedance of the power supply part 3 and the power reception part 5 is matched) such that the best transmitting efficiency is obtained under a condition in which a center axis Z1 of the power supply side helical coil 33 and a center axis Z2 of the power reception side helical coil 51 are aligned in a line with a lateral displacement x=0.
However, in the power supply system 1 described above, it is difficult to stop the motor vehicle 4 such that the center axis Z1 of the power supply side helical coil 33 and the center axis Z2 of the power reception side helical coil 51 are coaxially-positioned. Thus, as shown in FIG. 15, the lateral displacement x (>0) of the center axis Z1 with respect to the center axis Z2 may be formed.
For the power supply system 1 according to the conventional product shown in FIG. 15 in which the impedances of the power supply part 3 and the power reception part 5 are matched in a state in which the lateral displacement between the center axes Z1, Z2 is x=0, the inventors simulated the transmission efficiency for the lateral displacement x ranging from 0 to 0.375D (D=the diameter of the power supply side helical coil 33 and the power reception side helical coil 5. The result is plotted with black circles in FIG. 4.
As shown in FIG. 4, there is a problem that, while the transmission efficiency is about 98% when the lateral displacement x is equal to 0, the transmission efficiency is decreased to 82% when the lateral displacement x is 0.375D.
Furthermore, as shown in FIG. 16, another power supply system 1 has a capacitor C1, C2 connected to both ends of the power supply side helical coil 33 and the power reception side helical coil 51. The capacitors C1, C2 are provided for adjusting the resonant frequency and are set to a value which can obtain the desired resonant frequency f0 in accordance with the number of turns N in the power supply side helical coil 33 and the power reception side helical coil 51. In general, in this power supply system 1 provided with the capacitors C1, C2, the power supply side helical coil 31 and the power reception side helical coil 51 have the number of turns equal to 1.
However, in the above-mentioned power supply system 1 of the conventional product, there is a problem that, depending on a product, the transmission efficiency at the resonance frequency f0 is decreased. The inventors explored the cause of such. problem and found out that such problem is caused by the variations in the capacitors C1, C2. In general, it is guaranteed that a commercial capacitor has the capacitance of about plus or minus 5% to about plus or minus 10%. Thus, the variation in the capacitors C1, C2 is expected to be within this range. FIG. 17 shows the result of the simulation of the transmission efficiency around the resonance frequency f0 when the capacitance of the capacitors C1, C2 varies for plus or minus 5% and plus or minus 10%.
As can be seen in FIG. 17, the transmission efficiency at the resonant frequency f0 is equal to or greater than 90% for the conventional product a1 in which the capacitance C of the capacitors C1, C2 is a desired capacitance Cs with no error, and for the conventional products a2, a3 having the error of no more than about plus or minus 5%. On the other hand, the transmission efficiency at the resonant frequency f0 is decreased to about 50% for the conventional products a4, a5 having the error of plus or minus 10%.